FIELD-MEASUREMENTS OF WATER AND NITROGEN LOSSES UNDER IRRIGATED MAIZE

An intensive multidisciplinary experiment has been conducted over several years at La Cote Saint-Andre, near Grenoble, France. The major objective is to determine an optimal fertilizer application scheme for an irrigated agricultural system. Such a scheme would not degrade the quality of the environment, and yet would maintain a profitable level of crop production. This study is explicitly related to the cultivation of irrigated maize, a major crop in the area. The various terms of the water balance (consumption, drainage, soil storage) and of the nitrogen cycle (mineralization, plant uptake, leaching) were obtained from intensive monitoring in the upper layer of the 0.8 m of soil which corresponds to the root zone of the crop. This entailed the combined use of a neutron moisture meter, tensiometers and soil suction cups. To determine the specific effects of fertilization and crop growth, there were different treatments. These corresponded to a traditional fertilizer application of 260 kg N ha-1, no fertilization, and bare soil, carried out within an area of approximately 2 ha. Several sites were instrumented on each treatment, one of them being specifically for the application and the monitoring of N-15-tagged fertilizer. The results have shown that, in terms of the water balance, irrigation water management is extremely efficient, as drainage losses under the maize culture are negligible during the crop cycle. The situation is totally different, however, during the intercrop period (October-April), owing to rainfall. Then the soil is left bare and evaporation is very small, and now the drainage corresponds to about 90% of total inputs from precipitation. In terms of the nitrogen cycle, the results showed clearly that up to 150 kg N ha-1 was produced by mineralization in the soil. Nitrogen leaching beyond the root zone during the crop cycle is negligible, regardless of the rate of fertilizer application, as a result of the very small amount of drainage, despite irrigation. A very important contrast was found, however, between the fertilized and unfertilized treatments at harvest. There was a residue of 182 +/- 64 kg N ha-1 in the fertilized sites, but none for the others. The whole quantity remaining in the root zone at harvest was then totally leached by winter rains. To decrease the risk of groundwater pollution, a reduction of about 100 kg N ha-1 from the traditional application rate has been recommended. Finally, the method of estimation of N balance has been successfully validated by a comparison between N uptake determined by direct analysis of the whole plant and the value estimated from the temporal variations of the N content in the soil.